PCH: add recirculation rate as a metric to the sidebar

[danielabloch]

Is your feature request related to a problem?
As a content creator, making sure what I write deepens engagement with our overall business is a top priority. One of the best ways for us to measure that engagement is with recirculation rate.

Describe the solution you'd like
When I'm preparing something for publication, I'd like the PCH to help me

1. find related top posts with high recirculation rates
2. show me after I publish what the recirculation rate is for the post I just created

Describe alternatives you've considered
The alternative is that I have to find this information manually in the main Parse.ly dashboard. That takes me away from the work of publishing and wastes time I don't have to waste.

Additional context
Validation for this idea came recently on a customer demo of the work in progress.

[acicovic]

I think that we're waiting on the Parse.ly API on this one. Let's post a message here when we know the API is ready!